1. Field of the Invention
The present invention relates to the general field of metal founding. More specifically, the present invention relates to continuous casting machines of the type having an upright, slowly rotating casting wheel with a peripheral groove forming a mold cavity and a flexible band encircling a desired segment of the casting wheel to close the mold cavity while molten metal is solidified.
2. Prior Art
The general type of continuous metal casting machine with which the present invention is concerned is disclosed in Properzi U.S. Pat. No. 2,710,433, issued June 14, 1955. An upright casting wheel has a peripheral groove forming a continuous annular mold cavity. A continuous band or belt is looped around both the casting wheel and a large idler-tensioning pulley so as to close the mold cavity for a desired segment of its circumference. Usually, the band engages the casting wheel to close the mold cavity on its downward run and departs from the casting wheel to open the mold cavity on its upward run. Molten metal is continuously introduced into the casting wheel groove at about the location that it is first closed by the band. The metal solidifies into a continuous bar which is extracted from the groove at about the location that the band departs from the casting wheel. Thereafter, the continuous bar can be milled into rolled metal rod.
In order to obtain high-quality rod, the continuous bar leaving the casting wheel must be substantially free of internal cracks and voids. According to Properzi Pat. No. 2,710,433,
[I]t is necessary to reduce to a minimum the path of solidification, that is to say the length confined between the cross-section of the bar not yet completely solidified and the one completely liquid. This path constitutes the piece of bar in course of formation, in which the linear shrinkage of solidification may produce considerable cracks. PA1 Fey et al. No. 2,749,584, issued June 12, 1956; PA1 Properzi No. 2,865,067, issued Dec. 23, 1958; PA1 Bray et al. No. 3,318,369, issued May 9, 1967; PA1 Bray et al. No. 3,319,700, issued May 16, 1967; PA1 Properzi No. 3,411,565, issued Nov. 19, 1968; PA1 Cofer et al. No. 3,454,077, issued July 8, 1969. PA1 Unbalanced cooling of the molten metal will result in a defective ingot structure, the ingot being subject to crystallization malformations, segregations, cracks, surface roughness and brittleness etc. and these defects being more noticeable according to increase [in size] of the ingot section. PA1 [N]o effective countermeasure has been taken or proposed to avoid the internal cracking, because the cause of the internal cracking has not been clarified.
Column 2, lines 34 to 40. Consequently, Properzi proposed to subject the casting wheel to "vigorous cooling by means of water circulating inside the peripheral ring [casting wheel]." Column 2, lines 41 to 42.
Nevertheless, in order to increase production it is desirable to increase the speed of the wheel which inherently increases the length of the so-called "path of solidification" and which necessitates more rapid extraction of heat from the metal transported by the casting wheel. Several U.S. patents are concerned with improved cooling systems of which the following are representative:
Each of the above patents discloses a casting wheel with internal cooling passages adjacent to the peripheral casting groove. In addition, in the known systems coolant such as water is sprayed onto the mold-closing band so that heat is extracted from all sides of the forming bar simultaneously.
Cofer et al. U.S. Pat. No. 3,279,000, issued Oct. 18, 1966, discloses the general type of cooling system now used, namely, closely spaced nozzles for spraying liquid coolant under high pressure against all sides of the casting wheel as well as against the band. The "Continuous Casting Wheel With Improved Cooling Arrangement" disclosed in Properzi U.S. Pat. No. 3,529,658, issued Sept. 22, 1970, also utilizes external spray cooling. That Properzi patent emphasizes that:
Column 1, lines 63 to 68. According to that patent, a complicating factor is the unavoidable shrinkage of the metal during solidification which causes it to separate from the walls of the surrounding mold cavity, thereby interfering with rapid and uniform conduction of heat for extraction by the cooling sprays. Properzi in such U.S. Pat. No. 3,529,658 proposed a novel mold designed to contract as the metal solidifies so that surface contact between the cast bar and the mold would be maintained. Another, somewhat similar approach has been to apply pressure to the band to flex it inward and thereby urge the solidifying continuous bar into contact with the base of the casting wheel groove.
Since about 1970 more sophisticated efforts have been made to control cooling in continuous casting machines to achieve high production rates without unduly reducing bar quality. In U.S. Pat. No. 3,626,479, issued Dec. 7, 1971, Properzi proposed monitoring the temperature of the casting wheel at a location prior to introduction of the molten metal and adjusting the cooling of the wheel accordingly.
Nighman in U.S. Pat. No. 3,642,055, issued Feb. 15, 1972, proposed a spray-cooling arrangement but with a porous wire mesh belt such that the coolant would pass through the belt into intimate contact with the metal in the casting wheel groove.
Lenaeus et al. in U.S. Pat. No. 3,774,669, issued Nov. 27, 1973, described "zone cooling" in which different cooling rates are applied at different stages of solidification of the metal, recognizing that an air gap forms between the metal and the casting wheel during the final phases of solidification which interferes with heat conduction. During such final phase, Lenaeus et al. proposed to withdraw the mold-closing band for direct spray-cooling of the cast bar while still in the casting wheel groove.
Properzi in U.S. Pat. No. 3,800,852, issued Apr. 2, 1974, and U.S. Pat. No. 3,916,984, issued Nov. 4, 1975, also proposes "zone cooling" mechanism in which sprays at different locations along the circumference of the wheel can be adjusted independently of sprays at other locations.
Another aspect of the conventional manner of cooling the continuous cast bar is disclosed in Ward U.S. Pat. No. 4,069,860 which discusses the problem of heat transfer through the mold-closing band. High heat-transfer rate materials, such as thin metal strips, typically have lower structural strength. Consequently, the useful life of the band must be balanced against its heat-transfer rate. The suggestion of the Ward patent is to provide an "ablative material" on the inner surface of the band which material is charred or burned away due to the heat of the molten metal so as not to interfere with heat-transfer through the band.
Richards in U.S. Pat. No. 4,122,889, issued Oct. 31, 1978, proposed to increase cooling at the outer surface of the continuously cast bar adjacent to the band by injecting fluid between the band and the outer surface of the bar. According to Richards, the fluid under pressure also would produce an inward-directed force to seat the bar in the casting groove and thereby prevent formation of the air gap caused by shrinkage.
Kimura et al. U.S. Pat. No. 4,194,553, issued Mar. 25, 1980, is of interest because it recognizes the problem of internal and surface cracks in the continuous cast bar and proposes mechanism for guiding the solidified cast bar from the casting wheel. Still, according to Kimura et al.,
Column 2, lines 56 to 59.
Sinha et al. in U.S. Pat. No. 4,552,200, issued Nov. 12, 1985, and No. 4,588,018, issued May 13, 1986, propose a continuous casting system in which a "steady state" condition is established with cooling at maximum capacity. Thereafter, the speed of the casting wheel is adjusted while the location of final solidification and the porosity of the extracted cast bar are monitored in an attempt to achieve maximum production with acceptable bar quality.
Despite the considerable effort, imagination and research expenditures represented by the systems of the patents described above, the search has continued for ways to increase production and to increase bar quality--preferably both--in continuous casting machines.